1. Field of the Invention
This invention relates to a ball screw mechanism to transform form rotational/linear movement to linear/rotational movement and, an electric power steering apparatus using such ball screw mechanism.
2. Discussion of the Background
A ball screw mechanism that transforms rotational movement into linear movement is used for an electric power steering apparatus in order to transmit assist power of the electric motor to a rack shaft that is disposed between right and left wheels so as to steer the wheels by movement thereof in its axial direction. Such ball screw mechanism consists of a screw shaft which a thread groove is formed on its outside surface, a nut which a thread groove is formed on its inside surface and plural balls which exist between the thread grooves. More particularly, the balls are disposed to be able to roll with receiving road into a spiral ball-roll path that is created by opposing the thread groove of the screw shaft and the thread groove of the nut each other. A ball-return path to cycle the balls endlessly is formed into the nut. When the nut is rotated, rotational force of the nut is smoothly transformed into force that moves the screw shaft in its axial direction by the balls being cycled endlessly through the ball-roll path and the ball-return path according to the rotation of the nut. As a result the screw shaft is moved in its axial direction. Similarly, when the screw shaft is rotated, the nut is moved in its axial direction, and when one of the screw shaft and the nut is moved in its axial direction, the other is rotated.
As the ball-return path formed into the nut, for example, a construction using a return tube that connects both ends of a thread groove of a nut, and a construction that thread grooves formed into each end cap are connected by a ball-return path that is formed into a nut along its axial direction are known. A deflector-type ball-return path is adapted for the electric power steering apparatus. That is, a deflector that a passage, which a ball can pass, is formed into is attached into a through hole which goes through a nut in its radial direction so as to carve a part of a thread groove out. Therefore, the deflector and the part of the thread groove that is cut off create a ball-return path by attaching the deflector to the through hole. A ball screw mechanism using the deflector-type return-path has plural deflectors which are arranged to depart from each other with same distance in both of the axial direction and the circumference direction. Plural ball-roll paths that the balls are endlessly cycled in are created, which a number of ball-roll paths is same as the number of the deflectors. Length of the ball-roll path is equivalent with one round of spiral in this ball screw mechanism, because of characteristic of the ball-return path. Since the weight of the nut is balanced, this ball screw mechanism is suitable for the electric power steering apparatus that the screw shaft is made a rack shaft of the electric power steering apparatus and that the nut is turned by an electric motor.
When the nut is rotated for the screw shaft relatively, the balls endlessly move along the ball-roll path as the balls roll themselves. Then, because the balls contact to the next ball each other, frictional force that disturbs the rolling of the balls occurs at the contacting point of each ball. The frictional force not only disturbs the operation of the ball screw mechanism but also causes generation of noise and deterioration of lifetime. Meanwhile, each ball undertakes load in the ball screw mechanism. Generally, it is preferable that number of the balls is many, because load capacity of the ball screw mechanism becomes large. Therefore, it is preferable that distance between balls next to each other is short in order to arrange many balls.
According to the foregoing, a ball screw mechanism that separators are arranged between each balls to prevent contacts of balls and to keep distances between the balls short is known (for example, by Japanese Laid-Open Patent Application No. 2000-120825 and No. 2000-199556). The separator is a ring-shape or a disk-shape whose diameter is smaller than the same of the ball. A concavity that a part of the ball fits into is formed on each end face of the separator so that the part of the ball next to each other is slidably fitted into. As a result, the separators are held by the balls next to each other. Small clearance is configured between each ball and each separator in this state. The clearance is decided to be satisfied that the balls are permitted to roll properly and prevented to slide for the separator unnecessarily and that the separator that is assembled last can be assembled between the ball which was assembled first and the ball which was assembled last. Therefore, each separator is held by balls next to each other so as to keep the clearances, does not fall down, and moves together with the ball along the ball-roll path.
However, at the above-described ball screw mechanism, there are dispersions of lengths of the ball-roll paths, because of manufacturing error. Several kind of separator whose widths are different each other are prepared and selectively used for adjusting the clearances between the balls and the separators. Therefore, the parts management of the separator is complicated. Further, a process that selects suitable separator must be added in assembling the ball screw mechanism. Especially, since the ball screw mechanism using the deflector-type return-path has plural ball-roll paths, the adjustment of the clearance is needed for each ball-roll paths.
At the ball screw mechanism using the separator, it is possible that the separator catch on a seam between the ball-roll path and the ball-return path or on a portion that the direction of the ball-roll path changes. This causes torque non-uniformity and the balls being blocked up. It is remarkable at the ball screw mechanism using the deflector-type return-path, because a gap is easily created at a seam between the ball-roll path and the ball-return path. As a provision, separators whose periphery surface is formed spherically in order to avoid being caught in are known by the aforementioned Japanese Laid-Open Patent Application No. 2000-120825. However, it is difficult to machine the separator spherically, because the separators are extremely small parts.
Therefore, the electric power steering apparatus that adopted the above-described conventional ball screw mechanism becomes a high cost because of the complicated parts management and the extra assembling process. In addition, there is a problem that it is possible that operation failures of the electric power steering apparatus happen because of the torque non-uniformity or the ball being blocked.